1. Field of the Invention
The present invention relates generally to an apparatus and method for receiving data in a mobile communication system using an Adaptive Antenna Array (AAA) technology and more particularly to an apparatus and method for receiving data using a reception beam weight generation technique.
2. Description of the Related Art
A next generation mobile communication system is evolving for providing packet service communication systems that transmits burst packet data to a plurality of mobile stations (MSs). The packet service communication systems are suitable for transmission of mass data. Such a packet service communication systems are designed for high-speed packet service. In this regard, the 3rd Generation Partnership Project (3GPP), a standardization organization for asynchronous communication technology, has proposed High Speed Downlink Packet Access (HSDPA) to provide the high-speed packet service, while the 3rd Generation Partnership Project 2 (3GPP2), a standardization organization for synchronous communication technology, has proposed 1× Evolution Data Only/Voice (1×EV-DO/V) to provide the high-speed packet service.
Both HSDPA and 1×EV-DO/V propose to provide high-speed packet service for smooth transmission of Web/Internet service, and in order to provide the high-speed packet service, a peak throughput as well as an average throughput should be optimized for smooth transmission of packet data as well as circuit data such as voice service data.
In order to support the high-speed transmission of packet data, a communication system employing HSDPA (hereinafter referred to as an “HSDPA communication system”) has recently introduced 3 kinds of data transmission techniques: Adaptive Modulation and Coding (AMC), Hybrid Automatic Retransmission Request (HARQ), and Fast Cell Selection (FCS).
The HSDPA communication system increases the data rate using the AMC, HARQ and FCS techniques. Another communication system for increasing the data rate is a system which uses 1×EV-DO/V (hereinafter referred to as a “1×EV-DO/V communication system”). The 1×EV-DO/V communication system also increases a data rate to secure system performance.
Aside from the new techniques such as AMC, HARQ and FCS, there is a another technique which is known as the Multiple Antenna technique which is suitable for coping with the limitation in assigned bandwidth, i.e., increasing the data rate. The Multiple Antenna technique can overcome the limitation of bandwidth resource in a frequency domain because it utilizes a space domain.
The Multiple Antenna technique will be described hereinbelow. A communication system is constructed such that a plurality of mobile stations communicate with each other via a single base station (BS). When the base station performs high-speed data transmission to the one or more mobile stations, a fading phenomenon occurs due to a characteristic of radio channels. In order to overcome the fading phenomenon, a Transmit Antenna Diversity technique (TADt) which is a version of the Multiple Antenna technique, has been proposed.
The TADtis a method for transmitting signals using at least two transmission antennas, i.e., multiple antennas, to minimize a loss of transmission data due to a fading phenomenon, thereby increasing the data rate. The TADt will be described herein below.
Generally, in a wireless channel environment of a mobile communication system, unlike that of hard-wired channel environment, a transmission signal is actually distorted due to several factors such as multipath interference, shadowing, wave attenuation, time-varying noise, interference, etc. Fading caused by multipath interference is closely related to the mobility of a reflector or of a user (or of a mobile station), and is actually, a mixture of a transmission signal and an interference signal which are received at the same time.
Therefore, the received signal suffers from severe distortion during its actual transmission, reducing performance of the entire mobile communication system. The fading may result in distortion of both the amplitude and phase of the received signal, preventing high-speed data communication in the wireless channel environment. Therefore research is being conducted in order to resolve the fading caused by multipath interference.
In conclusion, in order to transmit data at high speed, mobile communication system must minimize a losses due to characteristics of mobile communication channels such as fading, and interference.
In order to prevent unstable communication due to the fading, a diversity technique incorporating multiple antennas is used to implement a Space Diversity technique.
The TADt is popularly used for efficiently resolving the fading phenomenon. The Transmit Antenna Diversity receives a plurality of transmission signals that have experienced independent fading phenomena in a wireless channel environment, thereby coping with distortion caused by the fading. The Transmit Antenna Diversity is classified into Time Diversity, Frequency Diversity, Multipath Diversity, and Space Diversity.
In other words, in order to perform high-speed data communication, a mobile communication system must well cope with the fading phenomenon that severely affects communication performance. The fading phenomenon must be overcome because it reduces amplitude of a received signal from between several dB to tens of dB.
In order to overcome the fading phenomenon, the above diversity techniques are used. For example, Code Division Multiple Access (CDMA) technology adopts a Rake receiver that can achieve diversity performance using delay spread of the channel. The Rake receiver is a type of Receive Diversity technique for receiving multipath signals. However, the Receive Diversity used in the Rake receiver suffers disadvantages because it cannot achieve desired diversity gain when delay spread of the channel is relatively small.
The Time Diversity technique efficiently copes with burst errors occurring in a wireless channel environment using interleaving and coding, and is generally used in a Doppler spread channel. However, the Time Diversity technique can hardly obtain the diversity effects in a low-speed Doppler spread channel.
The Space Diversity technique is generally used in a channel with a low delay spread such as an indoor channel and a pedestrian channel which is a low-speed Doppler spread channel. The Space Diversity technique achieves a diversity gain using at least two antennas. When a signal transmitted via one antenna is attenuated due to fading, a signal transmitted via another antenna is received, thereby acquiring a diversity gain. There are two types of Space Diversity techniques which are commonly used: Receive Antenna Diversity which uses a plurality of reception antennas and Transmit Antenna Diversity which uses a plurality of transmission antennas.
Receive-Adaptive Antenna Array (Rx-AAA) is a subcategory of the Receive Antenna Diversity technique.
In the Rx-AAA technique operates by calculating a scalar product between a signal vector and an appropriate reception beam weight vector of a reception signal received via an antenna array comprised of a plurality of reception antennas. Signals received in a directions desired by a receiver are maximized and a signals received in directions not desired by the receiver are minimized. The “reception beam weight” refers to a weight with which the receiver generates a reception beam in the Rx-AAA technique.
As a result, the Rx-AAA technique amplifies only desired reception signals to their maximum level thereby maintaining a high-quality call and causing an increase in the entire system capacity and service coverage.
Although the Rx-AAA can be applied to both Frequency Division Multiple Access (FDMA) mobile communication systems and Time Division Multiple Access (TDMA) mobile communication systems, it will be assumed herein that the Rx-AAA is applied to communication systems which use CDMA (hereinafter referred to as a “CDMA communication system”).
FIG. 1 is a block diagram illustrating a structure of a base station receiver in a conventional CDMA mobile communication system. The base station receiver includes N reception antennas (Rx ANTs) (which include a first reception antenna 111, a second reception antenna 121, . . . , and an Nth reception antenna 131); N radio frequency (RF) processors (which include a first RF processor 112, a second RF processor 122, . . . , and an Nth RF processor 132, which are mapped to the corresponding reception antennas) respectively; N multipath searchers (which include a first multipath searcher 113, a second multipath searcher 123, . . . , and an Nth multipath searcher 133 which are mapped to the corresponding RF processors respectively; L fingers (which include a first finger 140-1, a second finger 140-2, . . . , and an Lth finger 140-L), for processing L multipath signals searched by the multipath searchers; a multipath combiner 150 for combining multipath signals output from the L fingers, a deinterleaver 160, and a decoder 170.
Signals transmitted by transmitters in a plurality of mobile stations (MSs) are received at the N reception antennas over a multipath fading radio channel. The first reception antenna 111 outputs the received signal to the first RF processor 112. Each of the RF processors includes an amplifier, a frequency converter, a filter, and an analog-to-digital (A/D) converter, and processes an RF signal.
The first RF processor 112 RF-processes a signal output from the first reception antenna 111 to convert the signal into a baseband digital signal, and outputs the baseband digital signal to the first multipath searcher 113. The first multipath searcher 113 separates L multipath components from a signal output from the first RF processor 112, and the separated L multipath components are output to the first finger 140-1 to the Lth finger 140-L, respectively. The first finger 140-1 to the Lth finger 140-L, being mapped to the L multiple paths on a one-to-one basis, process the L multipath components.
Because the L multiple paths are considered for each of the signals received via the N reception antennas, signal processing must be performed on N×L signals, and among the N×L signals, signals on the same path are output to the same finger.
Similarly, the second reception antenna 121 outputs the received signal to the second RF processor 122. The second RF processor 122 RF-processes a signal output from the second reception antenna 121 to convert the signal into a baseband digital signal, and outputs the baseband digital signal to the second multipath searcher 123. The second multipath searcher 123 separates L multipath components from a signal output from the second RF processor 122, and the separated L multipath components are output to the first finger 140-1 to the Lth finger 140-L, respectively.
In the same manner, the Nth reception antenna 131 outputs the received signal to the Nth RF processor 132. The Nth RF processor 132 RF-processes a signal output from the Nth reception antenna 131 to convert the signal into a baseband digital signal, and outputs the baseband digital signal to the Nth multipath searcher 133. The Nth multipath searcher 133 separates L multipath components from a signal output from the Nth RF processor 132, and the separated L multipath components are output to the first finger 140-1 to the Lth finger 140-L, respectively.
In this way, among the L multipath signals for the signals received via the N reception antennas, the same multipath signals are input to the same fingers. For example, first multipath signals from the first reception antenna 111 to the Nth reception antenna 131 are input to the first finger 140-1. In the same manner, the Lth multipath signals from the first reception antenna 111 to the Nth reception antenna 131 are input to the Lth finger 140-L. The first finger 140-1 to the Lth finger 140-L are different only in signals input thereto and output therefrom, and are identical in structure and operation. Therefore, only the first finger 140-1 will be described herein for simplicity.
The first finger 140-1 is comprised of N despreaders of a first despreader 141, a second despreader 142, . . . , and an Nth despreader 143, being mapped to the N multipath searchers, a signal processor 144 for calculating a reception beam weight vector for generating a reception beam using signals received from the N despreaders, and a reception beam generator 145 for generating a reception beam using the reception beam weight vector calculated by the signal processor 144.
A first multipath signal output from the first multipath searcher 113 is input to the first despreader 141. The first despreader 141 despreads the first multipath signal output from the first multipath searcher 113 with a predetermined spreading code, and outputs the despread multipath signal to the signal processor 144 and the reception beam generator 145. Here, the despreading process is called “temporal processing.”
Similarly, a first multipath signal output from the second multipath searcher 123 is input to the second despreader 142. The second despreader 142 despreads the first multipath signal output from the second multipath searcher 123 with a predetermined spreading code, and outputs the despread multipath signal to the signal processor 144 and the reception beam generator 145. In the same way, a first multipath signal output from the Nth multipath searcher 133 is input to the Nth despreader 143. The Nth despreader 143 despreads the first multipath signal output from the Nth multipath searcher 133 with a predetermined spreading code, and outputs the despread multipath signal to the signal processor 144 and the reception beam generator 145.
The signal processor 144 receives the signals output from the first despreader 141 to the Nth despreader 143, and calculates a reception beam weight set wk for generation of a reception beam. Here, a set of first multipath signals output from the first multipath searcher 113 to the Nth multipath searcher 133 will be defined as “xk.” The first multipath signal set xk represents a set of first multipath signals received via the first reception antenna 111 to the Nth reception antenna 131 at a kth point, and the first multipath signals constituting the first multipath signal set xk are all vector signals. The reception beam weight set wk represents a set of reception beam weights to be applied to the first multipath signals received via the first reception antenna 111 to the Nth reception antenna 131 at the kth point, and the reception beam weights constituting the weight set wk are all vector signals.
A set of signals determined by despreading all of the first multipath signals in the first multipath signal set xk will be defined as yk. The despread signal set yk of the first multipath signals represents a set of signals determined by despreading the first multipath signals received via the first reception antenna 111 to the Nth reception antenna 131 at the kth point, and the despread signals constituting the despread signal set yk of the first multipath signals are all vector signals. For the convenience of explanation, the term “set” will be omitted, and the underlined parameters represent sets of corresponding elements.
Each of the first despreaders 141 to the Nth despreaders 143 despreads the first multipath signal xk with a predetermined despreading code, so that the reception power of a desired reception signal is higher than the reception power of an interference signal by a process gain. Here, the despreading code is identical to the spreading code used in transmitters of the mobile stations.
As described above, the despread signal yk of the first multipath signal xk is input to the signal processor 144. The signal processor 144 calculates a reception beam weight with the despread signal yk of the first multipath signal xk, and outputs the reception beam weight wk to the reception beam generator 145. As a result, the signal processor 144 calculates the reception beam weight wk including a total of N reception beam weight vectors applied to the first multipath signal xk output from the first reception antenna 111 to the Nth reception antenna 131, with the despread signals yk of a total of N first multipath signals output from the first reception antenna 111 to the Nth reception antenna 131. The reception beam generator 145 receives the despread signals yk of a total of the N first multipath signals xk and a total of the N weight vectors wk. The reception beam generator 145 generates a reception beam with a total of the N reception beam weight vectors wk, calculates a scalar product of the despread signal yk of the first multipath signal xk and the reception beam weight wk corresponding to the reception beam, and outputs the result as an output zk of the first finger 140-1. The output zk of the first finger 140-1 can be expressed as Equation (1):zk=wkHyk  (1)
In Equation (1), H denotes a Hermitian operator, i.e. a conjugate-transpose. A set zk of output signals zk from L fingers in the base station receiver is finally input to the multipath combiner 150.
Although only the first finger 140-1 has been described, the other fingers operate equivalently to the first finger 140-1 in operation. Therefore, the multipath combiner 150 combines the signals output from the first finger 140-1 to the Lth finger 140-L, and outputs the combined signal to the deinterleaver 160. The deinterleaver 160 deinterleaves the signal output from the multipath combiner 150 in a deinterleaving method corresponding to the interleaving method used in the transmitter, and outputs the deinterleaved signal to the decoder 170. The decoder 170 decodes the signal output from the deinterleaver 160 in a decoding method corresponding to the encoding method used in the transmitter, and outputs the decoded signal as final reception data.
The signal processor 144 calculates a reception beam weight wk such that a Mean Square Error (MSE) of a signal received from a mobile station transmitter, desired to be received by a predetermined algorithm, is minimized. The reception beam generator 145 generates a reception beam using the reception beam weight wk generated by the signal processor 144. The process of generating a reception beam so that MSE is minimized is called “spatial processing.”
Therefore, when Rx-AAA is used in a CDMA mobile communication system, temporal processing and spatial processing are simultaneously performed. The operation of simultaneously performing temporal processing and spatial processing is called “spatial-temporal processing.”
The signal processor 144 receives multipath signals despread for each finger in the above-stated manner, and calculates a reception beam weight capable of maximizing a gain of the Rx-AAA according to a predetermined algorithm. The signal processor 144 operates to minimize the MSE.
Reception beam weight calculation algorithms for adaptively minimizing the MSE, arealgorithms for reducing errors on the basis of a reference signal, and support the Constant Modulus (CM) and the Decision-Directed (DD) techniques as a blind technique, when there are no reference signals.
The signal processor 144 calculates a reception beam weight capable of maximizing a gain of the Rx-AAA technique according to a predetermined algorithm by receiving multipath signals after despreading for an individual finger as described above. Algorithms for calculating a reception beam weight capable of maximizing the gain of the Rx-AAA technique, include a Maximum Signal to Noise Ratio (Max SNR) technique and a Least Mean Square (LMS) technique.
1) Max SNR
Max SNR is an algorithm for maximizing output power of a received signal. In Max SNR, output power is calculated according to Equation (2):
                                                        P              =                              E                ⁡                                  [                                      zz                    H                                    ]                                                                                                        =                                                                    w                    _                                    H                                ⁢                R                ⁢                                  w                  _                                                                                        (        2        )            where R=E[yyH]. As illustrated in FIG. 1, ‘z’ denotes a value after despreading, to which a weight vector is applied, and ‘y’ denotes a value given before it is multiplied by the weight vector.
In this case, Max SNR can be expressed as Equation (3):
                              max          ⁢                                          ⁢                                                    w                _                            ⁢              R              ⁢                              w                _                                                                                      w                  _                                H                            ⁢                              w                _                                                    ,                              subject            ⁢                                                  ⁢            to            ⁢                                                  ⁢                                          w                _                            H                        ⁢                          w              _                                =          1                                    (        3        )            
An operation process of calculating the weight is defined as                1. Estimate w0,r0,v0         2. k=k+1                    a. R=f*R+x(k)·xH(k) <N2+N>            b. λ=wH(k)Rw(k) <N2+N>            c. r(k)=λw(k)−Rw(k) <N>            d. q(k)=∥r(k)∥2/∥r(k−1)∥2 <N>            e. v(k)=r(k)+q(k)v(k−1) <N>            f. a=wH(k)Rv(k) <N2+N>                            β=vH(k)Rv(k) <N2+N>                γ=wH(k)v(k) <N>                δ=vH(k)v(k) <N>                a=βγ−δα                b=β−λδ                c=α−λγ                t(k)=[−b+(b2−4ac)1/2]/2a                                    g. w(k+1)=w(k)+t(n)v(n) <N>            h. Normalize w(k+1) <1.5N>                        3. Iterate step 2 for a new snapshot        
In this process, < > means the number of calculations, and it can be understood that in Max SNR, the total number of calculations is O(4N2+11.5N). That is, the number of calculations is in proportion to O(4N2+11.5N). Here, ‘O’ means ‘order’, and a value N means one complex calculation. For each complex calculation, calculation for real parts or imaginary parts are performed four times. That is, complex calculation of (a+jb)(c+jd) is achieved by four multiplications of ac, ad, bc, and bd.
2) LMS (Least Mean Square)
LMS (Least Mean Square) uses a gradual update technique so that a cost function calculated on the basis of the MSE is minimized. Such an algorithm is superior in performance, but inferior in implementation due to a great number of calculations. That is, like in Max SNRmany (i.e., O(4N2+11.5N) calculations are performed for each complex calculation, causing an increase in the hardware complexity of the reception apparatus.